1. Field of the Invention
This invention relates to a method of and an apparatus for checking the four-wheel steering characteristics of a four-wheel steering system for a vehicle in which the rear wheels are arranged to be turned in response to the turning of the front wheels when the steering wheel is turned.
2. Description of the Prior Art
There has been proposed a four-wheel steering system in which both the front wheels and the rear wheels are turned in response to operation of the steering wheel. The four-wheel steering system is advantageous over the conventional two-wheel steering system in various aspects. For example, when the rear wheels are turned in the same direction as the front wheels (same phase) during high speed travel of the vehicle, the steering performance is improved to facilitate smooth lane changing since side force simultaneously acts on both the front and rear wheels. On the other hand, when the rear wheels are turned in the direction opposite to the turning direction of the front wheels (reverse phase) during low speed travel of the vehicle, cornering performance is improved and the minimum turning radius of the vehicle is reduced.
The preferable rear wheel turning direction (the direction in which the rear wheels are turned for a given turning direction of the front wheels, that is, in the same direction, i.e., the same phase, or the reverse direction, i.e., the reverse phase) and the rear wheel turning angle ratio (the ratio of the turning angle of the rear wheels to the turning angle of the front wheels), vary depending on various factors such as the running conditions of the vehicle. (These characteristics are referred to as "the four-wheel steering characteristics", hereinbelow.) Accordingly, based on various concepts, there have been proposed various four-wheel steering systems in which the running condition of the vehicle is automatically detected and the four-wheel steering characteristics are automatically changed according to the running condition of the vehicle.
For example, it may be generally considered that the front wheels are not turned by a large angle when the vehicle runs at a high speed, and the time the front wheels are turned by a large angle is solely when the vehicle runs at a low speed. Accordingly, in one proposal, when the front wheel turning angle is relatively small, the rear wheels are turned in the same direction as the front wheels in order to, for instance, improve running stability, and when the front wheel turning angle is relatively large, the rear wheels are turned in the direction opposite to the front wheel turning direction in order to, for instance, improve cornering performance.
In another proposal, the vehicle speed is directly detected, and the four-wheel steering characteristics are controlled according to the vehicle speed.
In other proposals, load on the vehicle, torque distribution in a four-wheel drive vehicle having a torque distribution changing mechanism, inclination of the road, friction coefficient of the road, vehicle altitude, steering wheel operating speed, road gripping condition of the tires, and lateral acceleration acting on the vehicle are respectively detected and the four-wheel steering characteristics are controlled according to these factors.
Further, the rear wheel turning mechanism may be mechanically connected to the front wheel turning mechanism so that the rear wheels are turned in response to operation of the steering wheel, or the rear wheel turning mechanism is electrically controlled so that the rear wheels are turned in response to operation of the steering wheel.
See, for example, U.S. Pat. Nos. 4,552,239, 4,601,357, 4,610,328 and 4,621,702, and Japanese Unexamined Patent Publication Nos. 61(1986)-108070, 62(1987)-8869, 62(1987)-8871, 62(1987)-8872, 62(1987)-12472, 62(1987)-18367, 62(1987)-12471 and 60(1985)-166561.
In the conventional two-wheel steered vehicle, the front wheels are turned by an angle proportional to the steering wheel turning angle, and accordingly, adjustment of the normal straight-ahead position of the front wheels with respect to the neutral position of the steering wheel suffices. However, in the case of the four-wheel steered vehicle, the rear wheels are turned on the basis of predetermined four-wheel steering characteristics. Accordingly, whether the rear wheels are turned correctly on the basis of the predetermined four-wheel steering characteristics must be checked in order to ensure running stability of the vehicle.